The present invention generally relates to a thermal head, and more particularly, to a thick film type thermal head having improved resistance against abrasion and wear.
Conventionally, there has been known a thick film type thermal head T as shown in FIG. 9, which includes an insulative substrate 12 made of alumina ceramics and the like, and an under-glaze layer 13 formed on said insulative substrate 12 as a heat accumulating layer by printing an amorphous glass paste onto said insulative substrate 12 for subsequent baking or sintering (referred to as baking hereinafter).
Moreover, on the under-glaze layer 13, an electrode 14 is formed by printing gold (Au) paste thereon for baking, with subsequent etching for pattern formation. In a manner to overlap said electrode 14, a resistor paste is printed for subsequent baking to form a heat-generating resistor 16.
Furthermore, over the insulative substrate 12, an overcoating layer 17 is formed so as to cover said heat-generating resistor 16 and the electrode 14 thereby. This overcoating layer 17 is also composed of an amorphous glass, and is formed by printing and baking a glass paste in the similar manner as in the under-glaze layer 13.
It is to be noted here that the above overcoating layer 17 has an effect to prolong the printing life of the thermal head by improving abrasion resistance of said thermal head with respect to a thermosensitive paper and a transfer ribbon.
In the conventional thermal head T as described so far, owing to the arrangement that the under-glaze layer 13 is formed prior to the overcoating layer 17, baking temperature at the baking process of said overcoating layer 17 is required to be set lower than a softening point of the under-glaze layer 13.
On the other hand, the relation between optimum baking temperature and the glass hardness after the baking is such that the hardness is reduced as the baking temperature is lowered, and from the viewpoint of maintaining a sufficient hardness of the overcoating layer, it is necessary to bake the glass paste for the overcoating layer at as high a temperature as possible within a range not exceeding the softening point of the amorphous glass for the under-glaze layer.
Meanwhile, in the relation between the baking temperature of the glass paste for the overcoating layer and pin hole density, the glass paste for the overcoating layer is required to be baked at high temperatures as far as possible within the rang not exceeding the softening point of the amorphous glass for the under-glaze layer 13 also for realizing reduction of pin holes.
However, at the present stage, there has not been proposed as yet, any superior glass paste for the overcoating layer which is capable of being baked at as high a temperature as practicable with respect to the softening point of the amorphous glass for the under-glaze layer 13 and simultaneously, can provide sufficient hardness and low pin hole density for the thermal head.
As one of the measures to solve the problems as described above, it is intended to raise the hardness by mixing a filler, for example, of fine particles of alumina (Al203), into the glass paste for the overcoating layer, but there is a possibility that mixing of a large amount of the filler deteriorates smoothness on the surface of the overcoating layer 17 for consequent reduction of the printing quality.
Table 1 given below shows characteristics of three kinds of glass pastes A,B and C for the overcoating layer currently in use. It is to be noted here that in the above case, the softening point of the amorphous glass for the under-glaze layer 13 is at 950.degree. C., and the transition point thereof is at 690.degree. C.
TABLE 1 __________________________________________________________________________ A B C __________________________________________________________________________ Product name LS201 made LS223 made LS207 made by Tanaka by Tanaka by Tanaka Matsusei Matsusei Matsusei Component PbO 27.4 wt % 19.7 wt % 38.34 wt % SiO2 18.6 13.9 5.4 Al2O3 21.6 30.8 18.9 Filler (Al2O3) ZrO2 1.5 1.1 0.01 CdO 31.0 34.5 37.4 Hardness Hk 604 710 550 In the case of (kg/mm2) baking temp. at Surface 0.14 0.28 0.06 810.degree. C. roughness Ra (km) Good Bad Superior Softening point 750.degree. C. 750.degree. C. 550.degree. C. Hardness Medium High Low Pin hole Good Bad Superior __________________________________________________________________________
FIG. 4 shows the relation between the surface roughness of the glass pastes A,B and C, for the overcoating layers and the baking temperatures.
As is seen from Table 1 and FIG. 4, the surface roughness of the glass paste C with the filler at 18.9% is small as compared with that of the paste A or B. When the surface roughness of a first layer for the overcoating layer is small, the surface roughness of a second layer should naturally become small, with consequent improvement of the surface smoothness, whereby accumulation of paper dust or dirt due to contact of the thermal head with the paper during printing operation may be advantageously reduced for clear and definite printed characters so as to be suitable for a color printer or the like in which particular importance is attached to the image quality.
The glass paste C having a low softening point, with addition of only a small amount of the filler is superior in the aspect of the surface roughness, but low in the hardness. In the glass paste A having a high softening point and added with a small amount of the filler, each of the surface roughness, pin hole density and hardness is generally acceptable somehow. On the contrary, although the glass paste B having a high softening point and added with a large amount of the filler is superior in the aspect of hardness, it is inferior from the viewpoints of the surface roughness and pin hole density. Thus, even when any of the glass pastes A,B and C is employed, it is difficult to obtain the overcoating layer fully satisfactory.
Similarly, there has also been conventionally known a thick film type thermal head which is applied to printing of bar codes. Such a bar code printing thick film type thermal head is applied, for example, to an automatic ticket vending machine or the like for printing bar codes on railway tickets, etc. Since such automatic ticket vending machines, etc. are installed outdoors in many cases, sands and the like tend to be taken into the thick film type thermal head for the bar code printing etc., and moreover, due to the fact that paper for the railway tickets and the like to be handled by the vending machines is generally hard in quality, a so-called "scratch breakage" is readily produced on the thermal head through peeling off or separation of the overcoating layer.
As one means to overcome the disadvantages as described above, increase of hardness for the glass constituting the overcoating layer may be considered. However, glass with a high hardness tends to be rough on its surface. Moreover, increase of the hardness requires higher baking temperatures, but such baking temperatures for the overcoating layer are undesirably limited from the aspect that the overcoating layer is to be formed after formation of the under-glaze layer, electrode and heat-generating resistor. Accordingly, there is a limitation to the raising of the hardness of the overcoating layer, and thus, effective scratch breakage prevention measure can not be provided thereby.
Moreover, as another means for solving the above disadvantages, there may be employed a practice to increase the thickness of the overcoating layer, but this practice involves a new problem such as reduction in the printing efficiency, thus not providing any particularly effective prevention measure against the scratch breakage, either.